Electrophotographic printing systems commonly employ a rotating photoreceptor drum as a medium for receiving images to be printed. Typically, a portion of the surface of the photoreceptor drum is charged to high electric potential and then exposed to light to form the desired latent image. Rotation of the drum then carries the image-bearing portion of the surface past devices which develop the image and then transfer it to a permanent medium such as paper. Further rotation of the drum brings that portion of the surface past an erasing device to prepare it for charging and exposure to a new image.
Following transfer of the image to paper, the image portion of the drum surface can have an electric potential anywhere between -300 and +500 volts. In the prior art, the erasing device used a corona in an attempt to bring this potential to near zero volts at all points. A separate charging device, called the primary corona, was then used to bring the drum potential up to approximately +1300 volts in preparation for exposure to a new image.
While functional, such systems were not entirely satisfactory in use because the resulting potential on the drum prior to exposure varied somewhat depending on the exposure and charging history of the drum. When the operating margin for the drum potential was low, as was the case near the end of normal drum or developer life, the variations in drum potential could lead to the appearance of traces of prior images in later printing. This problem was particularly severe when one image had been repeated many times; traces of this image could be seen in later prints of other images.